Sen. Rand Paul declared victory Thursday after Attorney General Eric Holder assured him that the president cannot use a drone to kill a noncombatant American on U.S. soil -- an assurance Paul had sought during his 13-hour filibuster the day before.

"Hooray!" Paul responded when the letter was read to him for the first time during an interview with Fox News. "For 13 hours yesterday, we asked him that question, so there is a result and a victory. Under duress and under public humiliation, the White House will respond and do the right thing."

During his dramatic filibuster, which delayed a vote on CIA director nominee John Brennan, Paul had demanded the administration clarify the government's authority to kill on U.S. soil. The filibuster ended past midnight early Thursday morning. The Senate voted, 63-34, to confirm Brennan later Thursday afternoon.

Shortly before the vote, Holder sent a terse letter to Paul that read: "It has come to my attention that you have now asked an additional question: 'Does the President have the authority to use a weaponized drone to kill an American not engaged in combat on American soil?' The answer to that question is no."